1. Field of the Invention
The present invention relates to a mobile communication system, a mobile communication terminal, and a power control method used therefor, and a program therefor and, more particularly, to a power control method in a CDMA (Code Division Multiple Access) mobile communication terminal.
2. Description of the Prior Art
In a conventional CDMA mobile communication system, as shown in FIGS. 1 and 2, Up/Down/Hold of downlink transmission power from a base station (Node-B) 4 is controlled by a downlink transmission power control bit (UL-TPC (Up Link-Transmit Power Control) information) from a mobile communication terminal (UE: User Equipment) 3.
The mobile communication terminal 3 always checks whether the quality of downlink reception data satisfies the required quality defined for each service (“a” in FIGS. 1 and 2). If the quality satisfies the required quality, the mobile communication terminal 3 transmits Down information through UL-TPC information (“b” in FIG. 2). Otherwise, the mobile communication terminal 3 transmits Up information through UL-TPC information (“b” in FIG. 1).
The base station 4 receives UL-TPC information from the mobile communication terminal 3. If the UL-TPC information indicates “Up”, the base station 4 upgrades the downlink transmission power (“c” in FIG. 1). If the UL-TPC information indicates “Down”, the base station 4 downgrades the downlink transmission power (“c” in FIG. 2). The quality of downlink reception data in the mobile communication terminal 3 is maintained at the required quality by this feedback control.
In the above conventional mobile communication system, this technique is effective when a mobile communication terminal is communicating with one base station. When, however, a mobile communication terminal is performing soft handover communication with a plurality of base stations, i.e., a plurality of downlink wireless links are present, the following problems arise.
Even if a plurality of downlink wireless links are present, only one uplink signal is transmitted from the mobile communication terminal, and there is only one piece of UL-TPC information. Assume that while the mobile communication terminal 3 is communicating with a base station 4-1 and power control is normally performed between the mobile communication terminal and the base station 4-1, a base station 4-2 is added (soft handover), as shown in FIG. 3. In this case, since no power control for downlink transmission power is performed between the base station 4-2 and the mobile communication terminal 3, the power of the mobile communication terminal 3 may be inappropriate for satisfying the required quality.
In addition, since uplink transmission power from the mobile communication terminal 3 is maintained to satisfy the required quality in the base station 4-1 by the same mechanism as described above (i.e., does not satisfy the required quality in the base station 4-2), the base station 4-2 may not properly receive UL-TPC information from the mobile communication terminal 3 immediately after the start of handover.
According to conventional downlink transmission power control, the mobile communication terminal 3 Rake-combines reception data from the base station 4-1 and reception data from the base station 4-2, and checks whether the combined reception data satisfies the required quality, thereby determining UL-TPC information.
As described above, the following phenomenon occurs under the situation where downlink transmission power of the base station 4-2 is too high when viewed from the mobile communication terminal 3 and the base station 4-2 cannot properly receive UL-TPC information from the mobile communication terminal 3.
The reception quality in the mobile communication terminal 3 after Rake combination is dominated by high transmission power from the base station 4-2, and hence greatly exceeds the required quality. The mobile communication terminal 3 therefore transmits Down information through UL-TPC information (“a-2”, “b”, and “c” in FIGS. 3 and 4).
However, the base station 4-2, which cannot properly receive UL-TPC information, continues transmission at high transmission power without complying with the Down information, and hence the mobile communication terminal 3 keeps transmitting Down information through UL-TPC information (“d-2” in FIGS. 3 and 4).
In contrast, since the base station 4-1 can properly receive UL-TPC information, the downlink transmission power of the base station 4-1 keeps downgrading owing to Down information through UL-TPC information continuously transmitted from the mobile communication terminal 3 (“a-1”, “b”, and “c” in FIGS. 3 and 4) and downgrades to a level at which the mobile communication terminal 3 cannot receive any information (“d-1” in FIG. 4).
Even in a handover state with respect to the base stations 4-1 and 4-2, downlink communication depends on only the base station 4-2 (reception data from the base station 4-1 cannot be received), and uplink communication depends on only the base station 4-1 (the base station 4-2 cannot receive any reception data from the mobile communication terminal 3).
In such a phenomenon, any Rake combination gain based on handover cannot be obtained. In addition, since the reception electric field of one base station (the reception electric field of a channel used for handover determination, which is different from a channel used for communication) deteriorates, when the link with the base station is disconnected (handover is canceled), uplink or downlink communication dependent on the base station is stopped. As a consequence, the communication itself is disconnected.